A wireless communication system typically includes a number of base stations that are configured to provide wireless coverage areas in which wireless communication devices (WCDs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices (whether or not user operated), can operate. In turn, each base station could be coupled with a core network that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a WCD within coverage of the system could engage in air interface communication with a base station and could thereby communicate via the base station with various remote network entities or with other WCDs served by the base station.
Such a system could operate in accordance with a particular radio access technology, with air-interface communications from the base stations to WCDs defining a downlink or forward link and air-interface communications from the WCDs to the base stations defining an uplink or reverse link.
Over the years, the industry has embraced various “generations” of radio access technologies, in a continuous effort to increase available data rate and quality of service for end users. These generations have ranged from “1G,” which used simple analog frequency modulation to facilitate basic voice-call service, to “4G”—such as Long Term Evolution (LTE), which facilitates mobile broadband service using technologies such as orthogonal frequency division multiplexing (OFDM) and multiple input multiple output (MIMO). And most recently, the industry is now exploring developments in “5G” and particularly “5G NR” (5G New Radio), which may use a scalable OFDM air interface, advanced channel coding, massive-MIMO, beamforming, and/or other features, to support higher data rates and countless applications, such as mission-critical services, enhanced mobile broadband, and massive Internet of Things (IoT).
In accordance various radio access technologies, each base station could be configured to provide service on one or more carrier frequencies or “carriers.” Each carrier could be frequency division duplex (FDD), defining separate frequency channels for downlink and uplink communication, or time division duplex (TDD), defining a single frequency channel multiplexed over time between downlink and uplink use. Each frequency channel of a carrier may occupy a particular frequency bandwidth defining a range of frequency at a particular position (e.g., defined by a center frequency) in radio-frequency spectrum.
Further, on the downlink and uplink, each carrier could be structured to define various physical channels for carrying information between the base stations and WCDs. For example, the air interface could be divided over time into frames, each divided in turn into subframes and timeslots, and the carrier bandwidth could be divided over frequency into subcarriers, which could be grouped within each timeslot to define physical resource blocks (PRBs) in which the subcarriers can be modulated to carry data. Further, certain PRBs or other resources per subframe could be reserved for special use.
When a WCD enters into coverage of such a system, the WCD could discover threshold strong coverage on a base station's carrier and could responsively engage in signaling to connect with the base station on that carrier and to attach or register for service if appropriate. The base station could then serve the WCD, coordinating use of the carrier's air-interface resources on an as-needed basis. For example, when the base station has data to transmit to the WCD, the base station could allocate particular downlink air-interface resources to carry that data and could accordingly transmit the data to the WCD on the allocated downlink resources. And when the WCD has data to transmit to the base station, the WCD could transmit to the base station an uplink resource request, the base station could responsively allocate particular uplink air-interface resources to carry the data, and the WCD could then transmit the data to the base station on the allocated uplink resources.